Frangible mold core for metal casting, production and use

ABSTRACT

The invention concerns the production of frangible ceramic mold cores for metallic casting, their production and use. In general, by the conventional debindering of the green ceramic, a substantial mechanical weakening of the cast mold occurs. Thus is particularly true in the case of awkward component geometry, such that a destruction of fine structures or freely projecting mold parts can occur during casting. It is thus the task of the invention to provide a geometrically complex mold core of slip-ceramic for metal casting, which exhibits a sufficiently high structural stability, in order to survive removal from pre-molds, as well as to survive metal casting without damage, and which thereafter can be removed in simple manner from the cast part. The task is solved in that the mold core includes ceramic microparticles, as well as a stable outer skin in which the microparticles are joined with each other by sintered nanoparticles.

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention concerns the production of frangible mold cores for metal casting, and in particular frangible mold cores of green ceramic, their production and use.

2. Related Art of the Invention

This type of mold core is known for example from DE 38 84 613 T2.

The production of cast parts with recesses, cut-outs, undercuts and hollow structures places high demands on the manufacturing methods and the materials of the corresponding cast molds. Due to the high temperatures involved, ceramic casting molds are employed as a rule in the field of metal casting.

For production of ceramic casting molds, one frequently employs slipcasting, in which the mold is shaped by the pouring of liquid slip into a pre-form. A further frequently employed process is ceramic injection-molding, in which a shapeable ceramic mass is introduced into a pre-mold under pressure. The slip or ceramic mass are subsequently solidified by drying or, as the case may be, cooling, whereby a green ceramic is formed. In particular in the case of complex shaped cast molds with precision structures, cantilevered or partially freely projecting, problems can occur during subsequent metallic casting, which problems result from the insufficient structural strength of the green casting mold.

During the conventional de-bindering of the green ceramic, a substantial mechanical weakening of the mold generally occurs. Thus, in the case of awkward mold design geometry, it can be possible that during casting a destruction of the fine or precise structures or the free projecting parts occurs.

The problem of insufficient structural strength can, in principle, be counteracted by increasing the strength of the ceramic, for example, by ceramic firing (sintering, as described in DE 38 84 613 T2). This however has the serious disadvantage, that the casting mold can only be removed with great difficulty out of the cast part after casting. This is particularly the case when producing hollow structures, where the remaining ceramic material is accessible only with great difficulty.

SUMMARY OF THE INVENTION

It is thus the task of the invention to provide a geometrically complex mold core of ceramic slip for metal casting, which exhibits a sufficiently high structural strength in order to survive removal from the pre-mold, as well as to survive metal casting without damage, and which thereupon can be removed from the cast part in simple manner.

With regard to the casting mold to be produced, the invention is accomplished by the characteristics set forth in Patent claim 1. A process for production thereof as well as its advantageous uses are set forth in the characterizing portion of Patent claims 3 and 8. The further claims contain advantageous embodiments and further developments of the inventive mold core, process and use.

With regard to the mold core to be produced, the task is inventively solved in that the mold core includes ceramic microparticles as well as a stable external skin, wherein the microparticles are joined to each other by sintered nanoparticles.

Microparticles are particles with diameters in the micrometer range, while nanoparticles are particles with diameters in the nanometer range. The external skin has a thickness of a few layers of microparticles, that is, at most one millimeter. The sintered external skin provides the mold core with sufficient stability and structural rigidity to survive the removal from the pre-mold as well as the metallic casting process without damage. Internally the mold core is comprised substantially of loose microparticles, which partially could be lightly sintered without thereby however extending into the stability region of the external skin. After metal casting the cooled metal shrinks onto the mold core, whereupon tension due to compression results. This compression tension weakens the stability of the external skin sufficiently to be able to break it apart with a stream of water, and thereby making it possible to rinse the external skin as well as the loose internal area or material of the mold core out of the cast shape.

Besides this, by limiting the sintering to the thin external skin, the typical sintering shrinkage is reduced to a minimum, whereby a substantially improved dimensional stability of the mold core results.

In a preferred embodiment of the inventive mold core the microparticles and/or nanoparticles are comprised of refractory oxides, carbides or nitrides, in particular of the elements Al, Zr, Si, Mg, Ca, or Ti. Preferably the microparticles are comprised of zirconium silicate and the nanoparticles of silicone dioxide.

This type of material exhibits a sufficient thermal resistance, a good workability as well as suitable commercial availability and acceptable cost in suitable particle sizes.

Preferably at least two particle size classes of micro-particles are employed in the production of the slip, in order to achieve an improved volumetric packing (density).

This slip is preferably prepared or mixed with water, in particular de-ionized water, in order to preclude the influence of additives to the greatest possible extent. This type of additive includes, for example, dispersing agents, anti-foaming agents and anti-freeze. The latter can be added in order to prevent the formation of large ice crystals during the freezing of the slip, which could otherwise lead to open structure faults in the mold core.

The inventive task with regard to the provision of a process for production of a frangible mold core for metallic casting is solved by the following steps:

-   -   providing a pre-mold,     -   providing a slip, containing ceramic microparticles,     -   filling the pre-mold with slip,     -   freezing the slip to form a green mold core,     -   releasing the green mold core from the pre-mold,     -   freeze-drying the green mold core,     -   applying colloidal nanoparticles upon the green mold core,     -   heating for         -   off-gassing of organic components of the green mold core and         -   sintering the colloidal nanoparticles, with formation of a             stable outer skin on the mold core.

Therein, the filling of the mold core with slip is accomplished by pouring in, injecting or dipping. The application of the colloidal nanoparticles upon the green mold core occurs by the spraying or painting on of a dispersion, for example using a brush. Further alternatives include dusting or dipping in the liquid dispersion.

The organic components of the green mold core are primarily binders as well as small amounts of additives. Upon their off-gassing the microparticles remain in the inside of the mold core, essentially in loose arrangement. The holding together of the mold core occurs primarily by the external skin, which develops its stability via sinter bridges of the partially or completely melted nanoparticles between the microparticles. These sinter bridges exhibit sufficient strength in order to survive without damage the removal from the pre-mold as well as the metallic casting. However, they are weakened by the metallic casting and by the subsequently occurring cooling and compression tension in such a manner that the mold core can be rinsed out of the cast shape.

In a preferred embodiment of the inventive process, during heating a maximal temperature of between 750 and 1200° C., preferably 800° C., is reached, wherein the maximal temperature is maintained constant for 30 to 120 minutes, preferably 60 minutes.

With a maximal temperature and duration of this type, when using the above mentioned materials, a sintering of the nanoparticles is achieved, which ensures a sufficient stability of the mold core.

In a particularly preferred embodiment of the inventive process the maximal temperature is reached by a continuous, preferably linear, temperature increase between 100 and 150° C./h, preferably 120° C./h.

This insures an even and complete off-gassing of the organic components.

Preferably the off-gasses are vented of f during heating, since, depending upon their composition, they could cause an odor or even be harmful to health.

In a further advantageous embodiment of the inventive process the application of the nanoparticles occurs by spraying or brushing on a dispersion of the nanoparticles, preferably using an aqueous dispersion. This makes possible the formation of an even outer skin. An aqueous dispersion is advantageous with regard to an environmentally friendly processing and the subsequent off-gassing.

With regard to the use of a frangible mold core for metallic casting, in accordance with the invention the task is solved by the casting of components for internal combustion engines of steel, light metals or their alloys, in particular the fine or precise casting following a lost wax process, preferably aluminum fine casting. Besides this, the invention is suitable for the production of ceramic casting molds produced from multiple parts.

In the following the inventive mold core, the process for its production and its use is explained in greater detail on the basis of an illustrative embodiment:

First, a pre-mold is prepared for a wheel guard. This is comprised of an external shell of multiple aluminum segments, which are milled from a blank and screwed together, as well as an internal shell of 1 to 3 mm silicone halves, which are introduced, precisely fitting to each other, into the outer shell. All segments as well as the silicon shells are sprayed with silicon spray as a release agent, in order to facilitate the later removal.

Besides this, a slip is prepared. For producing the slip the components according to Table 1 are weighed in a mixing container and mixed with each other over a period of four hours in a jar crusher. For improving the mixing process ceramic balls with a diameter of 20 mm are added. TABLE 1 Composition of the Slip Absolute Weight Relative Weight Component (g) (Weight Percent) H₂O, De-ionized 408 9.75% Anti-freeze 135 3.23% Dispersing Agent 40 0.96% Microparticle I 2700 64.55% Microparticle II 900 21.52% Total 4183 100.00%

The anti-freeze agent is glycerin, purest, 87%. This serves to prevent the formation of large ice crystals during freezing of the slip.

As dispersing aide a 25% aqueous solution of polyammonium-methacrylate is employed, which is available commercially under the trade name Darvan® C. As microparticles, zirconium silicate particles of various particle size distributions are employed. The Fraction I has an average particle size of approximately two Am (Tradename: Ultrox Standard), the Fraction II of approximately 23 μm (Tradename: Zircon 200 mesh).

Shortly before the end of the mixing time 10 drops of anti-foaming agent (combination of liquid carbohydrates, silica, synthetic copolymers, and non-ionic emulsifiers (Tradename: Agitan 280) were added in.

The finished slip has a density of 2.7 g/cm³ as well as a water content of 11 wt. %. It is stored with stirring.

The prepared slip is poured evenly into the already prepared pre-mold.

Thereafter the slip is frozen to produce a green mold core. For this the pre-mold filled with the slip is cooled evenly in a refrigerating device to an even temperature of −40° C. and left there for three hours.

The next step is the removal of the green mold core out of the pre-mold. For this, first the outer aluminum segments are removed and thereafter the flexible silicon halves are pulled off.

The still moist green mold cores are stored until they are freeze-dried at −20° C., that is, significantly below the freezing point of the ceramic suspension.

For freeze-drying a commercially available freeze-drying device is provided. The device is pre-cooled prior to the introduction of the frozen, still moist green mold core. Therein the support surface for the mold core is pre-cooled to approximately −30° C., and the condenser (as the required heat sink for condensing out the produced water vapor) is cooled to approximately −75° C.

For the freeze-drying, a drying protocol specific for the mold core and for the slip is followed: for the wheel guard of the above described slip the pressure is dropped to approximately 100 Pa (basically, the pressure must be below the triple point of water at approximately 600 Pa, and the temperature must correspondingly be lowered to the freezing point of the slip which is below that of water). During freeze-drying the support surface or platform for the mold core is constantly “heated” to approximately −15° C., in order to compensate for sublimation heat loss, which otherwise would strongly cool the mold core and slow the drying process.

After freeze-drying the green mold core is weighed and heated in a drying chamber to approximately 60° C. and follow-up dried for approximately 60 minutes at this temperature. Thereafter the green mold core is again weighed. This occurs primarily for reasons of quality control, since an incomplete freeze-drying would cause shrinkage cracks and inhomogeneities in a subsequent drying in ambient atmosphere. In general, the measured weight difference can however be dismissed and can be considered more as an (expendable) proof of quality of the freeze-drying.

Thereafter, the colloidal nanoparticles are applied to the green mold core. For this, an aqueous dispersion of silicone dioxide-nanoparticles (Tradename: Syton® X30, manufactured by DuPont) is thinly brushed on with a brush. The particles have an average size of approximately 40 nm and represent approximately 30 wt. % of the dispersion.

Thereafter the green coated mold core is heated and sintered. For this it is heated with a rate of heating up of 20° C./h to a maximal temperature of 800° C. and held at this maximal temperature for 60 minutes. Thereafter it is evenly cooled to room temperature.

Most of the organic components decompose in the temperature range of 200-300° C. The therefrom resulting exhaust gasses are removed by evacuation or suction.

The nanoparticles form in this type of temperature profile sinter bridges between the microparticles, so that a stable outer skin of the mold core is formed.

The stable mold core is introduced into the mold and this is filled with a metallic melt. After casting, the cooled metal shrinks upon the mold core, whereupon pressure tension results. This pressure tension weakens the stability of the outer skin sufficiently to be able to break it up with a water spray or jet and therewith to rinse the outer skin and the loose inner material of the mold core easily from the cast part.

The inventive mold core and the inventive process for production thereof demonstrate themselves in the above described illustrative embodiment as particularly suited for metallic casting, in particular for aluminum precision casting, in the automobile industry.

In particular, substantial advantages with regard to quality of internal surfaces can be achieved therewith.

The invention is not limited to the above described embodiments, but rather can be broadly adapted.

It is thus conceivable, that in place of the two size fractions of microparticles of zirconium silicate a single size fraction of Al₂O₃- or SiC-microparticles can be employed.

The nanoparticles of SiO₂ can be replaced by those of TiO₂, Al₂O₃ or ZrO₂.

In place of glycerin as anti-freeze it is also possible to employ gelatine, agaragar, Agarose or ethylenglycol. 

1. A frangible mold core for metallic casting, containing ceramic microparticles, wherein the mold core exhibits a stable outer skin, in which skin the microparticles are joined to each other by sintered nanoparticles.
 2. The mold core according to claim 1, wherein the microparticles and/or nanoparticles are comprised of refractory oxides, carbides or nitrides.
 3. A process for producing a frangible mold core for metallic casting comprising the steps: providing a pre-mold, providing a slip, containing ceramic microparticles, filling the pre-mold with the slip, freezing the slip to form into a green mold core, removing the green mold core from the pre-mold, freeze-drying the green mold core, comprising: applying colloidal nanoparticles upon the green mold core, heating: for off-gassing organic components of the green mold core and, for sintering the colloidal nanoparticles, with formation of a stable outer skin of the mold core.
 4. The process for producing a mold core according to claim 3, wherein the heating reaches a maximal temperature of between 750 and 1200° C., and wherein the maximal temperature is maintained constant for 30 to 120 minutes.
 5. The process for producing a mold core according to claim 4, wherein the maximal temperature is reached by a continuous thermal increase of between 100 and 150° C./h.
 6. The process for producing a mold core according to claim 3 wherein off-gasses are vented during heating.
 7. The process for producing a mold core according to claim 3 wherein the application of the nanoparticles is accomplished by spraying or brushing or dipping with a dispersion.
 8. A process for casting components for internal combustion engines of steel or light metal, said process comprising: (a) providing a frangible mold core for metallic casting, the core comprising a core and an outer skin, wherein the core of the mold core contains ceramic microparticles, and wherein the skin is rendered more stable than the core by applying nanoparticles onto the mold core and heating sufficiently to partially or completely melt the nanoparticles thereby joining the microparticles to each other to form a stable outer skin on the mold core, (b) introducing said mold core into a mold to produce a ceramic casting mold, (c) casting a molten metal into said casting mold to form a cast component, (d) cooling the cast component to cause shrinkage and application of tension on said mold core, reducing the stability of said outer skin, and (e) breaking up and removing the mold core from said cast component.
 9. The process as in claim 8, wherein said ceramic casting mold is comprised of an assembly of multiple components.
 10. The mold core according to claim 2, wherein the microparticles are zirconium silicate.
 11. The mold core according to claim 2, wherein the nanoparticles are silicone dioxide.
 12. The process for producing a mold core according to claim 7, wherein the application of the nanoparticles is accomplished by spraying or brushing on, or dipping in, an aqueous dispersion of the nanoparticles. 